1. Field of the Invention
The present invention relates to detecting a tracer gas in the near-surface atmosphere. More specifically, the present invention relates to a system for remotely locating mineral deposits, petroleum and natural gas deposits, geothermal steam deposits and leaks in natural gas pipelines by detecting a tracer gas using a single laser beam.
2. Description of the Prior Art
In the early stages of man's quest for natural gas and petroleum, almost all drilling for underground deposits followed a pattern of selecting drill sites by the proximity to oil and gas seeps which could be perceived without the aid of any equipment. Soon, however, deposits causing these visible seeps became exhausted, and those who wished to prospect for petroleum had to begin to rely on other methods. Over the past century, other techniques have been discovered and developed employing, for example, the refraction seismograph, the reflection seismograph, the magnetometer, and the gravameter. Together with geological analysis, the use of these geophysical tools has helped the prospector in his seemingly impossible task of locating with some degree of accuracy an underground deposit.
Without any direct surface indication, however, such indirect geophysical methods of prospecting are necessarily time consuming, laborious, complicated and expensive. In addition, and perhaps most importantly, these techniques have only marginally improved the exploratory drilling success ratio, i.e., the number of successful wells drilled versus the total number of wells drilled, since the period after the exhaustion of deposits associated with visible seeps and before such geophysical methods were available.
It would therefore be an enormous improvement if means were found to detect the most tenuous of seeps which have so far remained undetected in order to prospect for underground deposits. In most cases, these seeps will be located above "fault trap" types of deposits since the fault allows for a relatively easy communication of light hydrocarbon gases from the deeply buried deposit to the surface. However, undetected seeps may also be located above stratographic, anticline, reef, upward unconformity, salt dome and pinch out types of reservoirs. Obviously, they may be more easily distinguished above shallow reservoirs and over reservoirs which have a relatively porous caprock.
Numerous ground-based methods exist for measuring the concentration of hydrocarbons entrained in the soil and those which exist in the near-surface atmosphere. These methods necessitate that measurements be performed on the ground, however, in the vicinity of the seep. Surveying large amounts of land through the use of such ground-based methods is expensive and time consuming.
To overcome this problem, prospectors have developed a technique for remotely detecting light hydrocarbon molecules as they may exist emanating from the ground, indicating the presence of a deposit. Thus, for example, U.S. Pat. No. 3,651,395 to Owen et al teaches that light hydrocarbon molecules may be emanating from an underground hydrocarbon deposit. Owen et al teach the detection of the light hydrocarbon molecules through the use of their microwave re-radiation characteristics. In this scheme, target molecules are bombarded with microwave radiation that comes from a microwave transmitter on board an airborne platform. The light hydrocarbon molecules, after being struck by the microwave energy, rebroadcast at a different frequency. The particular frequency shift is characteristic of the target molecule in question. The rebroadcast radiation may then be monitored at the airborne platform.
Problems exist with this method in that it is necessary to bombard the earth's surface with microwave energy. In order for the light hydrocarbon molecules to radiate sufficient energy, a great deal of energy must be transmitted from the airborne platform. Even then, the signal received from the molecules is very weak, introducing sensitivity problems.
U.S. Pat. Nos. 4,100,481 to Gournay and 4,132,943 to Gournay et al both teach the use of microwave energy to excite gas. U.S. Pat. No. 3,351,936 to Feder also teaches the use of electromagnetic waves (either radio frequency or microwave) to explore subterranean structures. Two different radar wavelengths which have different penetration characteristics are transmitted and reflections are detected.
U.S. Pat. No. 3,741,653 to Svetlinchny teaches the use of an aircraft as a base for a laser measurement system. This system is intended only to monitor ground contours and not to detect the presence or location of particular materials. However, the remote detection of gases employing lasers is taught in Murray, "Remote Measurement of Gases Using Discretely Tuneable Infrared Lasers" in Proceedings of the Society of Photo-Optical Instrumentation Engineers, Vol. 95, pp. 96-104 (1976). This article teaches the projection of two laser beams having different frequencies through a remotely located sample chamber. Radiation reflected from a topical feature is detected, and the differential amount of absorption between the two laser beams is employed as an indication of the concentration of the sample gas in the remotely located chamber.
U.S. Pat. Nos. 3,861,809 to Hall and 3,807,876 to Nakahara both teach the measurement of the amount of absorption of light of a particular frequency to determine the concentrations of particular gases.
It is known to those skilled in the art that trace gases having spectral absorption characteristics in the ultraviolet range can be detected remotely, during prospecting, through the use of differential absorption laser radar (DIAL) techniques. Ahmed et al, "Remote Monitoring of Gaseous Pollutants by Differential Absorption Laser Techniques", Environmental Protection Agency, EPA-600/2-80-049, for example, teaches the remote detection of sulphur dioxide and nitrogen dioxide in the ultraviolet spectral region. In addition, Alden et al, "Remote Measurement of Atmospheric Mercury Using Differential Absorption Lidar", Optics Letters, Vol. 7, No. 5, May 1982 teaches the remote detection of sulphur dioxide and nitrogen dioxide in the ultraviolet spectral region. Also, Alden et al teach the remote detection of atmospheric mercury using differential absorption laser radar. Browell et al, in "Airborne Differential Absorption Lidar System For Water Vapor Investigations", Optical Engineering, January/February 1981, Vol. 20, No. 1, pp. 84-90, teach the use of DIAL to detect water vapor. However, the disadvantages of these techniques stem from the fact that two separate laser beams must be used to perform the differential absorption measurement.
Using two separate laser beams to perform the differential absorption measurement severely limits the sensitivity of the measurement. It is also known to those skilled in the art that during each measurement, the turbulent effects of the atmosphere play a role in degrading the measurement sensitivity. As taught by Killinger and Menyuk, "Remote Probing of the Atmosphere Using a CO.sub.2 DIAL System", IEEE J. Quant. Elect. Vol. QE-17, No. 9, September 1981, introducing a distinct time delay between the time of travel of one laser pulse with respect to the time of travel of another laser pulse will increase the statistical noise of the measurement. Thus, the sensitivity achievable using this technique is limited, and such a time delay between the two laser pulses is necessary when operating a single laser differential absorption lidar (DIAL Lidar) in the "sequential" mode due to the recovery time of (the laser the laser can not fire fast enough).
As disclosed in U.S. patent application Ser. No. 531,729, now abandoned, a DIAL Lidar can also operate in the "simultaneous" mode. In this mode, two physically separate laser beams are sent out simultaneously. Thus, there is no time delay between them. However, this mode is generally not feasible for gases having their absorption spectra in the ultraviolet because of the difficulty of manufacturing spectral bandpass filters which will allow one but not both of the beams to pass through. Furthermore, even if the problem of manufacturing sufficiently workable bandpass filters could be solved, thus providing a means to discriminate between the two beams when they return to the measurement platform, there is still a disadvantage in operating with two physically separate laser beams in the ultraviolet in the "simultaneous" mode. When operating in either the sequential or the simultaneous mode, for example, beam pointing errors degrade the measurement. This effect is caused by the fact that the two pulses may propagate through slightly different beam paths and reflect off slightly different sections of the ground. Thus, the difference in their measured intensities may be due to other factors rather than due to the presence of the tracer gas in the atmosphere.
The use of two laser beams also constitutes a serious practical disadvantage because the apparatus necessary to perform the measurement has to consist of two physically separate laser systems. The cost of two laser systems is a serious disadvantage. In the case of operation in the simultaneous mode, the apparatus has to consist of two physically separate laser systems in order to be able to generate the two different wavelengths simultaneously. In the case of operation in the sequential mode, on the other hand, the apparatus may also consist of two separate laser systems because of the need to fire over time intervals very much smaller than existing laser repetition rates will allow one to fire over because of the need to minimize the measurement error associated with changes in intensity due to a turbulent atmosphere. Thus, it is desirable for the time interval between pulses to be very small, which heretofore has been impossible with a single laser because of the inability to fire a single laser fast enough (on time frames less than 100 microseconds).
Canadian Pat. No. 808,760 to Bradley et al teaches a DIAL technique employing a single laser simultaneously producing two narrow-band frequencies. However it is difficult to achieve flexibility in selecting the frequencies produced and to control the relative power levels at the two frequencies.
It is also known to those skilled in the art that hydrocarbon deposits, mineral deposits, and geothermal steam deposits can be located through the use of mercury vapor anomalies. These anomalies occur in the soil gas and in the near-surface atmosphere. As taught in the article to Kartsev, the mercury vapor anomalies may indicate the presence of a buried petroleum or natural gas deposit because mercury as an atom in the environment often becomes absorbed into organic materials in sedimentary basins, and the element often becomes associated with petroleum and natural gas in buried pools. Furthermore, as taught by D'Itri and D'Itri, Mercury Contamination: A Human Tragedy, Wiley & Sons, New York (1977), up to 1.4 million pounds per year of mercury are released into the atmosphere from the burning of fossil fuels such as coal, oil, and natural gas. Accordingly, discoveries of high concentrations of mercury vapor are often indicative of the presence of an oil or gas deposit, since mercury is trapped in the supergene enrichment process by recycling into carboniferous precursor beds.
Also, it is known to those skilled in the art that deposits of precious and base metals may be detected through the use of mercury vapor anomalies in the soil gas and in the near-surface atmosphere. Hawkes and Williston, "Mercury Vapor as a Guide to Lead-Zinc-Silver Deposits", Mining Cong. J., December 1962, for example, teaches that patterns of mercury vapor may exist above concealed mineral deposits which can be used as exploration targets in the search for ore bodies. It is also known to those skilled in the art that geothermal steam deposits can be located with the aid of mercury vapor soil gas surveys. Matlick and Buseck, "Exploration for Geothermal Areas Using Mercury: A New Geochemical Technique", Proc. Second U.N. Geotherm, Symp., Govt. Printing Office (1976), for example, teaches the advantages of this mercury vapor technique in prospecting for geothermal deposits.
Thus, there is a need for a method and apparatus to overcome the limitations of having to use two physically separate laser beams to make a DIAL lidar measurement in order to locate a tracer gas such as mercury vapor in the ultraviolet range such that petroleum and natural gas, geothermal steam, and precious and base metals deposits can be located. A better method and apparatus is also needed for locating the presence of leaks in natural gas pipelines from a remote location, for while the presence of leaks somewhere in the line can be ascertained by a measured pressure drop, locating the leak exactly is much more problematical.